1. Field of the Invention
The invention relates to a synchronization method for a run length-limited (1.7)-code with a code rate of 2/3, in which code, data bits of a serial data stream are subdivided into two-bit wide data words and transcoded into three-bit wide code words, a sequence of the three-bit wide code words between adjacent ones ("1") comprising at least one zero ("0") but at most seven zeros ("0") and in which for the phase-correct decoding of the three-bit wide code words, a given synchronization bit sequence is inserted in the data stream of the code words.
2. Description of the Related Art
(1.7)-codes are used as channel codes in magnetically recording/reproducing data so as to increase the recording density or reduce the error rate. The journal IEEE Transactions on Magnetics, Vol. MAG-12, No. 6, November 1976, pages 740 to 742, describes such a run length-limited (1.7)-code in which each one ("1") must be separated from a subsequent one ("1") by a given number of zeros ("0"). In a (1.7)-code, a minimum of one and a maximum of seven zeros ("0") may occur between subsequent ones ("1"). The one ("1") bits within a coded bit sequence characterize the position of flux changes. A channel coding of the data bits provided by a source coder into code bits is performed in such a way that the data bits are subdivided into words of two bits and are transcoded into three-bit code words. The code rate is thus 2/3.
U.S. Pat. No. 4,337,458 describes a method of coding a run length-limited (1.7)-code with a code rate of 2/3c in which the four possible two-bit data words of the values 00, 01, 10 and 11 are converted into three-bit code words. The following Table shows a corresponding basic coding:
______________________________________ Data Word Code Word ______________________________________ 00 101 01 100 10 001 11 010 ______________________________________
Since this basic coding at the limits of the code words would also generate a transition between two "1" having a one-bit distance only, an additional coding prescription is used for these cases. The following Table shows this corresponding additional coding:
______________________________________ Colliding Supplied Data Words Code Words Code Words current subsequent current subsequent current subsequent ______________________________________ 00 00 101 101 101 000 00 01 101 100 100 000 10 00 001 101 001 000 10 01 001 100 010 000 ______________________________________
This channel code thus utilizes only a word supply with the following five words: 101, 100, 001, 010 and 000. Due to the dependence of coding on the subsequent word, this code is also referred to as look-ahead code. Tables for coding and decoding of the (1.7)-code are shown in column 4 of U.S. Pat. No. 4,337,458.
Another form of coding and decoding of a (1.7)-code with a code rate of 2/3 is known from IBM Technical Disclosure Bulletin, Vol. 23, No. 9, February 1981, pages 4375 to 4378.
In order to re-transform the 3-bit code word sequence recorded on a magnetic carrier into a 2-bit data word sequence at the reproducing end, the word limits of the reproduced 3-bit code words must first be determined. It is known from U.S. Pat. No. 4,868,690 to insert a special synchronous word in the sequence of the 3-bit code words to be recorded and to record it together with the 3-bit code word sequences. In the relevant case, the special synchronous word consists of a sequence of a plurality of code words of the digital value 010. It is attempted, at the reproducing end, to determine the position with respect to time of the inserted synchronous word sequence so as to define the word limits of the reproduced code words for subsequent decoding. The synchronous word recognition is realized by means of a shift register and an AND-gate connected to the outputs of the shift register. Stringent requirements are imposed on the synchronization of the reproduced code words, because an erroneous synchronization by one bit length already leads to a total loss of data.